Automatic signal bias control means and apparatus



June 15, 1948. I spfl 2,443,434

AUTOMATIC SIGNAL BIAS UONTBOL MEANS APPARATUS Filed May 23, 1944 2 Sheets-Sheet l ATTORNEY R. M. SPRAGUE 2,443,434

2 Sheets-Sheet 2 AUTOMATIC SIGNAL BIAS CONTROL MEANS AND APPARATUS June 15, 1948.

Filed May 23, 1944 ATTOR N EY the signals being transmitted are the usual telegraph signals of the mark and space kind. In such systems, it is usual to employ at the receiver, some kind of filter on which these square waves are impressed, with the object of filteringout any extraneous noise frequencies beyond the useful pass-band representing the actual signals. Such filtering action however, very frequently destroys or suppresses the harmonic content of the original square-topped signal waves and leaves only the fundamental thereof. therefore, in such systems, to follow the filter with some kind of a peak limiter or the like, whereby the smoothly rounded fundamental frequency Waves can be reconstituted into equivalent squaretopped waves. I have found that if the transmitting oscillator or the local beat frequency oscillator at the receiver is subject to fortuitous ire-- quency drift, there results a change in the normal bias of the reproduced signal due to the fact that the positive half waves of the reconstituted square-topped waves are not of equal duration with the negative half waves. If the recorder or keying device to be controlled by these reconstituted waves is arranged to record shade variations equally on both sides of a mean shade value e. g., gray, this lack of symmetry in the reconstitutecl square waves is revealed as a change in the average density of the reproduction.

Fig. 1A shows the original wave shape of telefacsimile or telegraph signals which, for purposes of explanation, are assumed to represent a series of dots in the original subject matter. After these signals are passed through a lowpass filter at the receiver, they assume the shape shown in Fig. 1B. The signals of Fig. 1B are then applied in the usual way to a peak limiter or the like whereby they are reconstituted into square-topped waves as shown in Fig. 10. If in the meanwhile, the transmitter frequency has drifted, or if the frequency of the local beat oscillator in the receiving radio set has drifted, the base line B of Fig. 13, will no longer be symmetrical with respect to the positive and. negative excursions but will have a non-symmetrical appearance as represented for example in Fig. 1D. I have found that by employing a special form of capacity and rectifier network ahead of the limiter, the undesirable effects of frequency drift are automatically compensated for.

Referring to Fig. 2, there is shown in schematic form a telefacsimile system employing the invention, but with well-known parts of the system omitted for the sake of clarity. The picture or other subject matter I', to be transmitted, is wrapped around the usual scanning drum 2, which is rotated so that the picture is completely scanned in successive elemental areas in the wellknown manner by means of an elemental light spot from source 3. The reflected light excites the photocell 4, whose output is amplified in any suitable amplifying system 5. The amplified signals are then applied through a suitable switch 6 to a radio transmitter preferably of the carriershift type comprising a carrier source 1 arranged to generate a normal carrier frequency which will be referred to as the base frequency, and a frequency shifting arrangement 8, whereby the picture signals cause the frequency of the carrier to be shifted between predetermined limits. For a detailed description of a preferred form of carrier shift arrangement that may be used, reference may be had to U. S. application Serial No. 498,278, filed August 12, 1943, the disclosure It is necessary of which is expressly incorporated herein by reference. This method of carrier shift is to be distinguished from ordinary amplitude modulation of a carrier, wherein the carrier fundamental frequency remains and has simultaneously upper and lower side bands. The output from device 8 consists at any given instant of a substantially single frequency carrier which however can be varied in frequency. For example, if the subject matter is printed matter or other simple black and white matter, the carrier may have a frequency of 9999.6 kc., when a black area is being scanned, and when a White area is being scanned, the carrier may have a frequency of 10000.4 kc. It will be clear therefore, that with ordinary printed matter and the like the transmitted carrier will make a great number of excursions above and below its mean frequency during each complete transmission, the excursion in one direction from this mean frequency being equal to the excursion in the opposite direction.

At the receiving station, the shifted frequency carrier is applied to any well-known type of radio receiver, preferably of the beat frequency type, so as to produce beats representing the frequency excursions of the transmitted carrier. It will be understood of course, that the beat oscillator ID at the receiver is designed to operate at a frequency which when beat against the signal from oscillator I will produce signal frequencies to operate the succeeding equipment.

The beats from receiver 9 which for example can be 2000 c. p. s., and 3000 c. p. s., are passed though a suitable input filter and limiter and then through a discriminator ll whereby they are converted into corresponding pulsating D. C. signals. These signals are then applied to a lowpass filter I2 which suppresses the A. C. components and produces at its output variable amplitude D. C. signals corresponding to the original signals from cell 4. For a detailed description of a discriminator and filter arrangement that is suitable, reference may be had to U. S. Patent As pointed out above, the signals applied to the discriminator l I are substantially square-topped waves (Fig. 1A) rich in harmonics, but at the output of filter l2 they are substantially sinusoidal (Fig. 1B). In order to reconstitute the square-wave shape, the waves are applied to a peak limiting arrangement I3 or similar device which is so adjusted that for a small voltage above the base line B (Fig. 1B), the limiter is keyed on, and at a small voltage below the line B, it is keyed off; thus producing the square wave of Fig. 1C. This square wave can then be used to control the recording element or printing hammer of any well-known form of telegraph reproducer or facsimile reproducer.

So long as the oscillators l and Ill do not drift in frequency, the half waves of the reconstituted keying signal (Fig. 1C) are symmetrical, both as to amplitude and time duration. I have found however, that if the frequency of the transmitter or receiver oscillator drifts, it will result in a shifting of the base line B (Fig. 1B) for example to the dotted line B. The effect of this is that the limiter I3 will no longer operate symmetrically in the plus or minus directions, but rather will produce a wave form such as shown in Fig. 1D. In other words, in the example mentioned, the negative bias D will be much greater than the positive bias D. This will result in improper operation of the recording element or keyerof thedevice l4.

eng nes In (order to overcome these dlfiiculti'es; therezis interposed rbetwe'en :the (filter 1 2 :andthe zlimiter l3 'ai-special network (Fig. 2-) The :re'sistor :I-B represents the iload fresistoriin the output of the filter I2, the voltage across *which :isalso changing between :the :two transmitted values :corresponding to'th'e'signals fromcell *4. Onthe :positive'swings .o'f potential'through resistor I16, condenser "I '1 is charged through rectifier Hi :to the peak voltage-of the signal. The time constant. of resistor .I 6 and .condenser I! must belowsenough so that condenser I 1 will reach its full charge during the duration of the signal :pulse. The voltage across resistor li9during this positivaexcursion'of the signal, remains zero. At the same time, :because of the poling of i-rectifier'ZB, icondenser '21 transmits the positive voltage across resistor: I 6 'to-resistor-M. Whentth'e .voltageaacross resistor f I 6 restorestqnormal, 'the negative charge on'conde'nser I? is impressed across resistor 19. Thus, the' voltage-at point '23 swings equallyon either side'of zero with respect to ground at the values 'of the peak input signal voltages. When a negative voltage appears-across resistoril fi, the opposite eiiect occurs, so far as'e'lements 11, it, 1 9, and 2 1, t'll, and 22, are concerned. Resistors "24, 25, 'are-of equal value, therefore the voltage between-23 and 'ground will always vary-equally in 'theplus and minus directions. 'The time-constant of condenser l1 and resistor #9 must be such that the charge on condenser I'l' will not leakofi? appreciably through resistor 19 between the extremepulses, and likewiseforcondenser?! ,and're'sistor -Z-2. The point 2-3 is-connected to the limiter 1'3 in such a way that-for small changes of voltage in the positive direction, the limiter is keyed on and for-small changes of voltage in the negative direction the limiter is keyed off. When no voltage appears across resistor It or when *the voltage across resistor 16 is constant, the voltage at point 23 with respect to groundis'zero. The limiter'lfi can then beadjusted "softhat this zero or quiescent condition holds the local circuit of the telegraph keyer 26 or recording element of the facsimile reproducer M in -e'ithermark" or space position 'asde- *sired. "The mean or average voltage 'across resistorTfi will not disturb this balancdbecause no direct current is transmitted'through condensers .l"! 'or2l. Therefore, it does "not matter whether the voltage across resistor B is varying about-zero or'abouta plus or negative'value. 'Only changes involtagesimpressed across resistor [6 determine the extremes of the'voltages at point 23. Thus,

the transmitter or.receiver can'drift any amount,

resultingin any'mean voltage across resistor '15 without disturbing the .balance. The circuitwill operate equally well when only occasional pulses are being received rather thandots of equal on and off duration. In Morse signalling or teletype -'-signals, the signals are far from symmetrical in regards to the on and ofi durations. Animportant application of this circuit is in teletype operation, wherethe slightest amount of bias (lengtheningorshortening of the marl: duration atthe expense'of th'e'space) is extremely harmful.

As above pointed 'out, where the telefacsim'ile "subjectmateris black-a'nd white, the transrmitted carrier isstatisticallyshifted betweenits "extreme frequencies. However, if the subject matter I is 'a"variable 'densityor shade photograph, some means must be provided to shift the carrier between its extreme frequency excursions a multiplicit of times during the transmission. This can be effected by providing contrasting Eblackf, :and'afiwhite?strips li-fieon-t el-drum, thus a ,ibla-ckfz and. iwhiteu strip :be vanterposed :between-ithe; spaced edges at the 221 101311116 ll whichJiswrappediaroundztheidrum. iGonseguently, .cat mach-:rotation :ofithe drum,- ?the merrier iirom the ;.:transmitter :is' :shifted bzetween' its (extreme frequency zexcu-rsions. In 1311630553 'of "telegraph such .as iteletype :or iMorse signal transmission as above described, where only itwonextremesrof signal conditions are encountered,- the shifting oi the "carrier .takesplace, automatically during transmission, Inrthecaseofawariablezshadesubjectmatter therefore, the :point 23 instead iof being connected through .;a ilimiterr l3, may control the recording Zlampu29 of any :suitable telephoto reco'rderl23. Under th'eseaconditions, when point 30 is in its quiescent icondition, .snffi'cient current "will --flow through the :recording :lamp 129 to :record mid-graywalues :of :shade,-"whereasithe two extremes in voltage appearing racross resistor l 6 -wi1l cause the voltage at point 12 3 to acorrespondingly vary between the blackest :blac'knnd the whitest white. The half tones transmitted between 'these'extremeiton'es will thenialways vary about the mid-gray valuaand a transmitter or receiver frequency "drift will not cause the average density of the "photo-reproduction to A var-y.

Various chan es and modifications may-he made "in the disclosed embodiments-without departing from the spirit "and scope of the invention.

What is claimed is: V

1. "Inan electric signalling system 'ot'the type emplo'ying'a carrier whose frequency is purposely varied with respect to a mean "frequency and between predetermined limits representing signails to be transmitted, means to receive said carrier and. ,including a local oscillator to detect saia signals, signal-reproducing means, circuit connections between said receiving means and said reproducing means :and including "a point "whose potential corresponds to "the said detected signals, and means between said receiving means and said point forvaryin'g the potential of said ,point. only corresponding to said .signals even though there exists undesired frequencydrift'in the saidmean carrier frequencyduring said signal detection, the said means "between said receiving means and said point comprising a capacity and rectifier. network whereby the potential of said point changes only when 'thesignal 'vdltages impressed .on said network are'yaryingrandindependently of frequency drift in said "local oscillator, said network including a .pair .of rectifiers .having a .commonreturn .and each connected 'throug'h respective condensers to the said receiving means, a, pair of rectifier load impedahces "having a common junction connected to-said common lreturn and an outputimpedancebridged across both said rectifier fload 'impeda'nces, said --.p.ointTbeing flocated,at 'the electrical m'idpointloi said. output, impedance.

'2. In an electric, signalling system .Offihe kind employing a carrier whose jfrequency is shifted symmetrically in opposite directions with respect 'toia mean frequency under control of signals :above .andfbelow a predetermined "level, receiving means of 'thebeatffrequency detector type for ,de-

testin Said signals, a filter, for the detected signals iioreliminating the frequency components outside a predetermined .band, a, network having an output point whose potential varies only when said detected signals vary, said network having a signal input point and including a pair of condensers and a pair of rectifiers oppositely poled with respect to ground and connected 'acrosssaid condensers,means to impress the detected signals between said inputpoint and ground, a load impedance connected-across said rectifiers with the electrical midpoint connected-to ground, another impedance connecting saidoutput'point symmetrically with respect to said first'impedan-ce, and a signal responder connected to said output point.

3. In a system of the character described, means to receive and detect signal voltages from a received frequency-shifted carrier, a signal reproducer, and a network between saidreceiving means and said responder for compensating for undesired frequency drift in the carrier, said network comprising a pair of conductors having their input ends bridged by a pair of condensers, and their output ends bridged by an output impedance, input impedance means to impress detected signals on the common junction point of said condensers, means connecting saidsignal responder to the electrical midpoint of said impedance, a pairof rectifiers bridged across said conductors adjacent said condensers, said rectifiers having a common return to said input impedance and being connected back-to-back with respect to said common return, a pair of impedances bridged across said conductors adjacent said first-mentioned impedances, said rectifiers and said pair of rectifier load impedances alsobeing connected to said common return, and said rectifiers being responder connected to said peak limiter, said network including a pair of rectifiershaving a common return and each connected through respective condensers to said receiver, a load impedance for each rectifier, said load impedances having a .common junction connected to said.

common return, and another impedancev bridged across both said load impedances and to the electrical midpoint of which said signal responder is connected.

5."In a signalling system, means to scan a subject matter having black and White areas to produce symmetrical and substantially squaretopped signal waves, a carrier generator, means to shift the frequency of said generator symmetrically around a mean frequency and between predetermined limits corresponding to said black and white areas, means toreceive and detect said signals and including 'a frequency discriminator and filter for the" detected signals, a frequency-drift compensatingv network having at least one input point and one outpoint point, said network including a pair of rectifiers'having a common return and connected back-to-back with respect to said common return, means connecting said rectifiers to the receiving means so that only the pulsating components of the detected signals are applied to said rectifiers'said 130 oppositely poled with respect to said common output being symmetrically connected to the combined output of saidrectifiers sothatsaid output point is maintained in a'quiescent potential condition even though there exist undesired carrier frequency drifts in the system.

t 6. In a signalling system, electro-opticalmeans to scan a picture or the like having a range of tone values between black and whitel .toipro duce electric picture signals, means to shiftthe frequency of a carrier between predetermined limits corresponding to the extreme ranges of said tone values, means independent of the'picture to subject the carrier to alternate frequency excursions between said predetermined 'limits, means to receive and detect the signals from said carrier, a picture shade reproducer, and-means between therreceiverand reproducer for reproducing a range of signal voltages corresponding to said picture signals'with the mean valueof the reproduced signals corresponding withthezmean value of the original signals and independently of any accidental drift in the mean frequency of said carrier, said last-mentioned means including a pair of rectifiers having a common returnand each connected through respective condensers to said receiver, a load impedancefor eachrectifier, said load impedances having a common junction connected to said common return, and another impedance bridged across both said-load impedances and to-the electrical midpoint ofwhich said shade reproducer is connected. 7 a

'7. A system according to' claim 6 in which the means for subjecting the carrier to alternate frequency excursions independently of the picture comprises longitudinal black and white strips which are scanned in cyclical recurrent intervals between the ends of each scanning line of the picture.

8. In a system of the character described, carrier frequency drift compensating means comprising a pair of rectifiers connected in parallel with an impedance, a common ground return connection between the junction of the said rectifiers and the electrical midpoint of said impedance, said rectifiers being oppositely poledlwith respect to said common return, a pair of condensers connected in series across said rectifieraa signal input impedance connected between the common junction of said condensers and said common return, wherebysignal voltagesare impressed in parallel on said condensers, a load impedance connected across said first impedance, a signal responder, and means connecting one terminal of said responder to the electrical midpoint of said load impedance and the other terminal of said responder to said commonv ground return.

ROBERT M. SPRAGUE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATESPATENTS 2,411,853 Finch Dec. 3, 1946 

